Zimbra email on Iphone

Hi Forum

I have one client who on several occasions has sent an email from their Iphone 4 & 5 with and without the latest IOS and magically the from field on the recipient side appears to be user@domainname@domainname. The problem obviously occurs when the recipient tries to reply and the address is invalid.
The email is configured using the exchange option on the iphone.
I can see in the zimbra webmail and the mail logs that it comes through as the funny address. The phone is attaching the domain name to the from field at end of the users email address for some reason.
Only one domain, multiple users of that domain and iphones only have experienced this issue. There is no difference in the domain settings as far a I can tell on the mailstores to any of our other domains.

Any ideas, or has any one else experienced this, or is it likely a problem with the programming on the Iphone.